1. Field of the Invention
This invention relates to a method and apparatus for phase modulation of the edges of clock or data signals. More particularly, the invention relates to such systems that add phase modulation to signals for the purpose of testing communications components and links.
2. Description of the Related Art
High-performance multi-gigabit (Gbit) per second communications links need to be implemented and tested using mechanisms that are cognitive of the important role that jitter, or phase modulation, causes in a system. When designing links and link components, it is often the case that jitter necessarily present in the signal is desired to be removed by a clock recovery unit specially designed for this task. When testing such devices or links, it is typical to purposefully create signals with high amounts of phase modulation (jitter) to be used as the stimulus during such tests. Whether it is to create a phase modulation inside a phase-locked loop for wide-bandwidth clock recovery or to cause phase modulation in a test stimulus, high-performance phase modulators with sufficient phase variation range, very high speed of modulation (modulation bandwidth) capable of operation for multi-gigabit per second data rate applications are needed.
Many mechanisms exist for phase modulation including phase interpolators, varacter-based variable delay lines, frequency modulating voltage-controlled oscillators, and limiting amplifier edge threshold modulation. Each has their drawback when compared to the three metrics just mentioned—range, bandwidth and data rate. It is desired to have range that approaches one-half of a data unit period, bandwidth of phase modulation injection that comes close to the bandwidth of the actual clock or data signal being modulated and to operate in application data rates up to and exceeding 12 Gbit/sec.
Phase interpolators are a mechanism that arithmetically adds one signal with an edge to another like signal with the same edge delayed by some prescribed amount (typically delayed by an amount that is less than or equal to the rise time). In this fashion, the output waveform is constructed as a sum as: Output=C1*Sig1+C2 Sig2 (where Sig2 is a delayed copy of Sig1). It is evident that if C1=1 and C2=0, then the output edge will be set by the timing of the edge in Sig1and that if C1=0 and C2=1, then the output edge will be set by the timing of the edge in Sig2. Smooth functions exist for choosing C1 and C2 between 0 and 1 that cause the output to have a smooth output delay variation from that of Sig1to that of Sig2. The limits of this approach for implementing phase variation of an output signal stem from the limited range of phase variation supported (limited by the rise time of the signal and the observation that low rise time will translate to added unwanted jitter) and the limited bandwidth possible. The limit in bandwidth in such an implemented system stems from how to apply C1 and C2 to the signals. This can be done with variable amplifiers or variable attenuators both of which do not offer as high a bandwidth control setting as desired.
Varacter-based variable delay lines can also be used to phase modulate a signal. In this approach, the physical observation that the delay of a transmission line is a function of the capacitance of the transmission line and that the capacitance of a transmission line can be voltage-controlled by using a semiconductor component called a varacter (a voltage-variable capacitor). In real implementation of such approaches, scores of varacters are needed to get the change in capacitance requisite to implement the desired range of variable delay. This causes the unwanted side-effect of having to drive a large capacitance at high frequencies when a high-frequency phase modulation is desired. Additionally, because the transfer function between voltage-in and phase variation (delay) out is non-linear requiring signal pre-conditioning in order to achieve the desired linear phase modulation, these types of variable delay technologies have natural limits in the bandwidth of phase modulation that can be supported which are well too low.
For some time now an alternative to phase modulation has been known. It is called sinusoidal frequency modulation. Such modulation is created using a sinusoidal frequency to modulate voltage controlled oscillators. Voltage controlled oscillators are requisite components of phased-lock loops found inside synthesizers and clock recovery units. It is well known to sum a modulation source into the control voltage of the voltage controlled oscillator to cause frequency modulation. There are natural limits to this method. It is known that any allowance for high-frequency modulation of the control voltage inputted to a VCO inside a phased-lock loop also allows noise to be modulated in the VCO, which translates to a large amount of unwanted jitter.
In actuality, the above known described method is really frequency modulation, supported by modulating a VCO's control voltage. This works as a natural limitation to this known method. This type of modulation is not the same as phase modulation except during sinusoidal modulation where the integral of a sine wave is a cosine wave which has a similar amplitude response. Thus, only sinusoidally shaped phase modulation is supported by this known method. As it turns out this is a very significant limitation. Therefore, known methods of frequency modulation of a VCO are severely limited in the type of phase modulation that can be supported.
More recently, attempts have been made to phase modulate clock or data edges using threshold modulation in limiting amplifiers as a means to accomplish high-frequency phase modulation. In this method, the edge of data or clock which is to be modulated (moved back and forth in time) is created by passing the edge through a differential limiting amplifier device. The limiting amplifier behaves in a way such that the output will transition from low to high when one of the differential inputs transition above the other (in voltage) and will reciprocally transition from high to low when one input transitions below the other. When no transitions are present, the limiting amplifier holds a fixed high or low voltage (depending on the comparison of the two differential input voltages). The finite rise-fall times of the applied data or clock edge, makes it possible for the movement of the comparing threshold “upward” (attached to one differential input of the limiting amplifier) to “push off” the point where the input signal (attached to the other differential input of the limiting amplifier) gets above the threshold which delays when the output of the limiting amplifier will change.
Similarly, moving the comparing threshold “downward” will “pull-in” the point where the input signal is above the threshold. This hastens the output of the limiting amplifier and causes it to change earlier than what would be expected. The ability to advance or delay the output edge of the limiting amplifier is sometimes termed an amplitude modulation to phase modulation translation. Changing the threshold amplitude applied as one of the differential inputs changes the output phase. The above structure benefits from the fact that both the inputs to a limiting amplifier typically have the same high-frequency bandwidth. Using this structure, very fast movement of the thresholds can be directly translated to very fast movement of the output phase. This structure is also able to implement linear-like phase modulation for a large percentage of an applied signal (as long as the edge rate of the applied signal is suitably slowed-down—after all, all the modulation range is set by the amount of walking up and down the applied signal edge that can be done on the modulation input). For instance, it is practical to modulate 25% to 30% of a sine-shaped applied input signal while staying within 2-5% linearity. However, it is observed that this technique suffers from the fact that rising edges and falling edges get the same threshold offset applied. The result is that a high threshold will delay a rising edge but will advance a falling edge. The net effect of this is that this approach has proper phase modulation for all rising edges or for all falling edges, but both edges end up creating a modulated duty cycle distortion, an undesirable effect.
Therefore, the method and apparatus in accordance with the instant invention overcomes existing limitations of range, data rate and bandwidth by creating a new phase modulation structure and method.